1. Field of the Invention
The present invention relates to a technical field of an optical pickup and a disc drive apparatus. More specifically, the present invention relates to an improvement of the quality of a tracking error signal by reducing the influence of eccentricity of a disc-shaped recording medium and the like.
2. Description of the Related Art
Disc drive apparatuses for recording/reproducing information signals on disc-shaped recording media are known. A typical disc drive apparatus includes an optical pickup that moves in a radial direction of a disc-shaped recording medium mounted on a disc table and irradiates the disc-shaped recording medium with laser light through an objective lens to record or reproduce information signals.
The optical pickup performs focus adjustment by detecting a focusing error signal and moving the objective lens toward and away from a recording surface of the disc-shaped recording medium (in a focusing direction) depending on the result of detection. In addition, the optical pickup performs tracking adjustment by detecting a tracking error signal and moving the objective lens in a substantially radial direction of the disc-shaped recording medium (in a tracking direction) depending on the result of detection.
A push-pull method is known as a method for detecting the tracking error signal. However, this method has a problem in that a large current-variation (DC offset) signal is easily generated when the objective lens moves in the tracking direction.
Therefore, a differential push-pull method, which can reduce the DC offset signal, is commonly used for detecting the tracking error signal (refer to, for example, Japanese Unexamined Patent Application Publication No. 61-94246).
In the differential push-pull method, laser light is divided into a main beam and a pair of sub-beams using a diffracting element. As shown in FIG. 13, spots of sub beams (sub-spots S) formed on the recording surface of the disc-shaped recording medium are positioned between the adjacent tracks T, so that distances from a spot of the main beam (main spot M) to the subs spots S are both one-half of a track pitch P.
In the differential push-pull method, the distances from the main spot M to the sub-spots S are both one-half of the track pitch. Accordingly, the phase of the tracking error signal detected by the main beam is inverted with respect to the phases of the tracking error signals detected by the sub beams, and the DC offset signal is canceled (see FIG. 14).